For G160M observations, please consult Chapter 9 of the COS Instrument Handbook to estimate overheads and the number of exposures per orbit, and consequently the correct lifetime position. Use of LP4 with G160M requires justification in the "Special Requirements" section of the Phase I PDF. Use of the default LP6 with G160M requires no justification. Proposals can be submitted with the warning present. For G160M observations, please consult Chapter 9 of the COS Instrument Handbook to estimate overheads and the number of exposures per orbit, and consequently the correct lifetime position. Use of LP4 with G160M requires justification in the "Special Requirements" section of the Phase I PDF. Use of the default LP6 with G160M requires no justification. Proposals can be submitted with the warning present. For G160M observations, please consult Chapter 9 of the COS Instrument Handbook to estimate overheads and the number of exposures per orbit, and consequently the correct lifetime position. Use of LP4 with G160M requires justification in the "Special Requirements" section of the Phase I PDF. Use of the default LP6 with G160M requires no justification. Proposals can be submitted with the warning present. 0 Assigned ID: 4551 ----- Attempting Submission 1 (Wed May 24 19:49:55 GMT 2023) ----- HST Phase I Proposal 4551 successfully submitted. Receipt: # 4551-1 ----- Attempting Submission 2 (Wed May 24 19:58:45 GMT 2023) ----- HST Phase I Proposal 4551 successfully submitted. Receipt: # 4551-2 ----- Attempting Submission 3 (Wed May 24 23:50:58 GMT 2023) ----- The only warning is regarding LP with G160M; we will use the nominal LP6 and thus do not require justification/explanation. INEFFICIENT ORDERING OF FP-POS POSITIONS 0 ----- Attempting Submission 1 (Thu Sep 14 19:19:31 GMT 2023) ----- We acknowledge the inefficient ordering warning but ignore it as the grating position changes occur during occultation and thus do not impact the available on-source integration times. The unexpected discovery of prominent lines of doubly- and triply-ionized nitrogen in one of the most luminous galaxies at z~10 has dramatically demonstrated both the power and the challenge represented by JWST spectroscopy. The spectrum of GN-z11 dramatically yields both a super-solar estimate of N/O and a measurement of the electron density orders of magnitude larger than anticipated for normal star-forming systems. Yet beyond these facts, the nature of this object remains hotly debated - in particular, whether such features necessitate a supermassive black hole or whether they can be produced by extremely concentrated metal-poor star formation remains unclear. As for many of the other surprising high-ionization emission lines found at high-z, galaxies in the local Universe may represent our best hope of casting empirical light on such distant detections - yet few constraints on these lines are available in the local UV archive. Here we propose unique COS spectroscopic follow-up of both the crucial NIV] and NIII] transitions in the UV at moderately-high resolution, in a sample of compact star-forming galaxies with strong evidence of substantial N/O enhancement nearby. These spectra will probe the limits of these emission lines produced by intense star formation today. We will leverage the constraints on both multiplets to investigate nitrogen production in very young star-forming environments near the metallicity of GN-z11, and probe gas conditions across densities orders of magnitude higher than accessible in the optical. This is a prime opportunity for direct dialogue between JWST and HST observations, which can only be realized while HST/COS remains operational. This program will acquire NUV+FUV spectroscopy designed to constrain high-ionization nebular emission in N III] 1750 and N IV] 1483,1386 (as well as other UV emission lines where possible simultaneously) for three metal-poor galaxies with signatures of highly-ionized gas and relatively high nitrogen abundances in the optical. The targets are all covered by SDSS imaging and spectroscopy by selection and have GALEX imaging useful in observation planning. Coordinates and acquisition: Each target is a compact source whose core (the target of our observations) is unresolved in ground-based imaging. The target coordinates are derived directly from the ICRS-aligned SDSS photometry. Following practices established in previous programs targeting similar objects, each target is acquired with a standard ACQ/IMAGE exposure, with exposure time chosen to deliver S/N=50 for a point source with an NUV AB magnitude equal to the faintest of the u-band fiber and cModel magnitudes (assuming a flat continuum in F_nu, appropriate for such sources). We confirm that all three have GALEX/NUV magnitudes in-excess of this conservative estimate; and to ensure detector safely, we likewise confirm that no warnings are raised for a Mirror A+PSA observation of a point source with the brightest GALEX/NUV magnitude among the three (~17.7). Based upon previous observations of similar sources, we expect these targets to be more extended than a point source, but with FWHM at NUV wavelengths significantly smaller than 0.6''; given this, the large requested S/N provides an appropriate buffer to ensure accurate acquisition of such targets. Observing plan: Our primary objective is nebular line measurement; our observations are thus in the low-S/N regime (<~5 /resel in the continuum). For each galaxy, we pick a grating setting that centers the crucial NIII] or NIV] lines of interest and additionally includes as many of the other nebular UV lines of interest (CIV, He II, OIII] in particular) as possible. Since these targets have also not been observed in the targeted UV regions prior, we adopt a conservative approach that 1) maximizes total exposure time on source while also 2) minimizing the likelihood of lost data due to partial buffer reads. Since we are in the low-SNR regime for all three targets, we only need to utilize2 FP-POS settings for G160M LP6 exposures and have no minimum for G185M. For the faintest source, J0240-0828, we are allocated 3 orbits per grating. To maximize integration time and minimize overheads, we adopt a long-dwell observing strategy that focuses on one grating per each of 2 visits and only changes the FP-POS between orbits. For J0240-0828, we fix the G185M observations at the bluest G185M/1890 FP-POS position (FP-POS=4) to capture as much of He II 1640 as possible. For the G160M visit, we observe at 3 FP-POS for better wavelength coverage (1,3,4). For the other objects, we simply observe at G185M FP-POS=3 and G160M FP-POS=1+4 (after confirming that the resulting small gap of unusable wavelengths in G160M then falls in a wavelength range that does not affect the target lines). The buffer time is chosen to maximize the amount of the buffer that is read-out while also minimizing overheads. The G160M exposures always have an estimated buffer fill time (BFT) an order of magnitude higher than the exposure times even with a conservative estimate of the spectrum (flat in AB with magnitude set to the GALEX NUV magnitude). The G185M exposure BFTs are close to the length of the longer integrations; since readouts occur during occultation and thus efficiency is not a concern, we simply choose a buffer time for these exposures to 1/2xBFT to ensure all data is read-out. Safety: The only BOT warnings generated are for the GSC2 check, which gathers the full light of each galaxy at V-band and assumes incorrectly that this light is in a O5V star. No GALEX BOT warnings are generated, even under the (conservative but incorrect) assumption that all of the GALEX NUV light is concentrated in an unresolved point source at COS resolution. The assembled team has extensive experience using HST/COS to study nearby dwarf galaxies with FUV+NUV spectroscopy. Over the past five cycles, members of our team have successfully led many proposals targeting similar samples of star-forming dwarfs with COS spectroscopy and published the results expediently (GO:14168, 14679, 15646, 15881, PI: Stark; GO: 17198, PI: Senchyna; GO: 13312, 14628, 15422, 15465, PI: Berg; and the CLASSY Treasury Program GO: 15840, PI: Berg, Co-PI Stark). Results from these programs have been published in Senchyna et al. 2017, 2019, 2021, 2022; Berg et al. 2016, 2019a,b, 2021, 2022; Chevallard+2018; and several in-preparation manuscripts. The proposed COS G160M observations are very similar in nature to the G160M data collected in these previous programs, and will be reduced and analyzed using the techniques we have developed and tested already. PI Senchyna and Co-I Stark are experienced with planning and analyzing HST/COS FUV spectroscopy of nearby dwarf galaxies, and will be responsible for data reduction and analysis. This will involve designing target acquisition and observing strategy, optimally combining exposures, and cleaning spectra of MW absorption lines. PI Senchyna and Co-Is Mingozzi and Berg are experts in the measurement and interpretation of high-ionization nebular emission lines, and will drive the measurement of the crucial transitions and derivation of density constraints. Co-Is Berg, James, and Mingozzi are experts in nebular abundance measurement and modeling, and will inform measurement of N/O as well as C/O and comparison to chemical evolution models. Co-Is Plat and Charlot have developed a full suite of both population synthesis + nebular photoionzation models and a robust model comparison framework, and will lead comparison of the nebular line strengths to those predicted by the population synthesis models constrained by the continuum data. They will also lead direct fits to the data using their tool BEAGLE (Chevallard & Charlot 2016), simulating and stress-testing techniques applied to lower-S/N data at high redshift. Co-I Stark is an expert in high-redshift observations, and their group will inform comparison of the observed spectra to high-redshift galaxies and implications thereof for ongoing JWST programs. 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